During the packaging of the semiconductor devices, it is typically necessary to place a semiconductor chip or integrated circuit die onto a substrate such as a leadframe, and then electrically connect the die and substrate with conductive bonding wires. In high-power integrated circuit packages, heavy aluminum wire is commonly used to make the connection and carry current between the die and the substrate. These aluminum wires typically have diameters of 5 mils and above, and can be as wide as 20 mils in diameter. The aluminum wires are preferably bonded to bonding pads of the respective die and substrate using wedge bonding.
There are several disadvantages associated with existing wedge bonding of heavy aluminum wires. Firstly, the cost of the heavy wire wedge bonder machines are expensive, which can be up to three times the cost of equivalent ball bonder machines. Secondly, the throughput of wedge bonding machines is very low, and the time it takes to bond a single wire by wedge bonding is up to three times longer as compared to an equivalent ball bonder machine. Hence, it makes economic sense for integrated circuit assembly houses to use copper wire instead of aluminum wire, because copper wire is cheaper and more suitable to ball bonding. Therefore, there are economic and other benefits to replace heavy aluminum wedge bonding machines with copper ball bonding machines.
In wedge bonding, since both the first and second bonds are formed in an identical manner, there is no substantial variation in the current-carrying capacity of the wire throughout the whole wire length as the cross-sectional area of the wire is about the same throughout the wire. Consequently, there is no significant difference in the pull strength of the first wedge bond at a first bonding pad, as compared to the pull strength of the second wedge bond at a second bonding pad. However, in ball bonding, the first bond is formed from a ball and the second bond is effected by pressing the wire between the capillary and the bonding surface resulting in a flattened area with diminished cross-sectional area now referred to as a stitch bond. The current-carrying capacity of the wire at a ball bond area of the first bond is thus higher than the current-carrying capacity of the wire at the stitch bond area of the second bond, where the cross-sectional area of the wire is at the lowest. There is thus a current-carrying bottleneck at the stitch bond. Furthermore, the smaller cross-sectional area means that the lowest bond pull strength of the wire is at the stitch bond.
Presently, copper ball bonding is generally confined to wire diameters of around 2 mils (about 50 microns) and below. For copper ball bonding of wires with wire diameters of more than 2 mils, the lack of stitch pull strength and non-uniformity of the wire causing greater electrical resistance at the stitch bond would pose greater operational issues. It would be desirable to increase the cross-sectional area of the wire at the stitch bond position so as to decrease the bottleneck effect and to increase the pull strength at the stitch bond position.
FIG. 1 is a cross-sectional side view of a capillary 100 according to the prior art. The capillary 100 has a capillary tip 102 that feeds a bonding wire 104 through a capillary hole 106 at the center of the capillary tip 102. There is a bottom face 108 at the base of the capillary tip 102 that is instrumental in pressing the bonding wire 104 onto a bonding surface. Adjacent to the bottom face 108 is a sloping capillary tip face 110, which leads to an outer radius 112 of the capillary tip 102. The sloping capillary tip face 110 forms a face angle A0 with respect to a horizontal bonding surface.
A stitch bond is formed by the capillary tip 102 deforming the wire 104 against the surface to be bonded, typically a “lead” or “second bond surface”, thereby producing a wedge-shaped bond. The top part of the stitch bond follows the contour of the sloping capillary tip face 110 and outer radius 112 of the capillary tip 102. The actual area welded or bonded under the stitch is dependent upon the capillary tip face design, the bonding parameters used (ultrasonic power, bonding time, bonding force and bond stage temperature) and the bondability of the material to be bonded to. A smaller face angle A0 of the sloping capillary tip face 110 will result in a stitch that is thinner than a sloping capillary tip face 110 with a larger face angle. Nevertheless, simply increasing the face angle A0 to obtain a thicker stitch would reduce bond strength and lead to an unreliable bond.
FIG. 2 is a cross-sectional side view of a stitch bond formed by the capillary of FIG. 1. The wire thickness X-X at a selected point of the stitch bond where the wire first meets the bonding surface is 39 microns. With this wire thickness for a heavy wire, problems with pull strength and current-carrying capacity as explained above may be experienced.
One prior art method of increasing the pull strength of the wire at the stitch bond position is described in Japanese patent publication number JP2001-291736 entitled “Capillary for Wire Bonding”. The publication discloses a capillary with a cone shape facing downward. A leading edge of the capillary is formed in two stages. The leading edge has a bottom face for leading out a fine wire, and an edge of the bottom face is used to cut the fine wire. There is also a step-like peripheral region which is located next to the edge. When the fine wire is cut by ultrasonic bonding, a part of the fine wire which is near the cut end is simultaneously pressed by the step-like peripheral region.
However, this invention is said to be applicable for bonding thin gold wires, specifically gold wires of 10-20 microns in diameter, not heavy wires. Although the step-like peripheral region with orthogonal orientations tends to press the wire at the wedge bond and deforms it to help in improving bond adhesion towards the end of the wire, the relatively flat contact at the bottom face of the capillary cuts the wire rather abruptly. Since the contact area between the stitch bond and the bonding surface is one of the factors that determine stitch pull strength, the flat bottom face of the capillary limits the contact area between the stitch bond and the bonding surface of the die or substrate. The end of stitch profile of the stitch bond is still relatively thin.
Moreover, the stitch bond that is formed has a stepped shape since the peripheral surfaces are parallel and perpendicular to the bottom face respectively, which decreases the uniformity of the wire. There may be a mechanical weakness formed in the bond because the sharp orthogonal edge of the step initiates micro-cracks in the stitch bond which will result in fractures during subsequent operational cycles. Sharp edges of the capillary will lead to very fast build up of wire material resulting in bonds with lower and inconsistent stitch pull strengths. Uniformity of current-carrying capacity at the stitch bond is lacking.